1. Field of the Invention
This invention relates to prostheses for use in treating back pain and more particularly to prostheses for replacing an intervertebral disc and associated intervertebral facet joints.
2. Background
Lower back pain is a very common disorder and is responsible for extensive morbidity and lost time at work. The prevalence rate of low back pain is very high, affecting approximately 80% of the general population at some time. Although most patients experience the painful symptoms only occasionally and recover fully, approximately 10% of these patients experience chronic and disabling low back pain in spite of various medical treatments.
The most common cause of chronic disabling low back pain is degenerative disk disease (DDD). Another problem associated with low back pain, which often accompanies DDD, is degeneration of the facet joints between vertebrae.
The functional unit of the spinal column is a spinal motion segment that is made of a three-joint complex, a disc anteriorly and two facet joints posteriorly. The facet joint is a synovial joint with the joint surfaces covered by hyaline cartilage like other diarthrodial joints. The orientation of the facet joints in the lumbosacral spine is symmetrical on both sides in almost all individuals, but it is occasionally found to be asymmetrical. Facet asymmetry has been reported in the literature to cause disc degeneration. The facet joint is generally oriented obliquely in sagittal and coronal planes. The orientation of the facet joints is significantly different at different levels within the spine, i.e., the cervical, thoracic, thoraco-lumbar, and lumbo-sacral regions of the spine. Within the lumbar or lumbo-sacral spine the size, shape, orientation, and angle of the facet joints have a wide range of variation among the motion segments at different levels within an individual. Such variations are even greater among different individuals.
The pathology of degenerative disease of the spinal motion segment commonly begins with intervertebral disc degeneration. Such degeneration of the intervertebral disc frequently results in abnormal biomechanics of the spinal motion segment, which leads, in turn, to degeneration of the facet joints. Symptomatic degeneration of the intervertebral disc and/or the facet joints of a spinal motion segment have been treated by surgical replacement of the nucleus pulpous of the intervertebral disc, the entire intervertebral disc, and/or the facet joints, with appropriate prostheses.
Replacement of the intervertebral disc with an artificial disc prosthesis is indicated for patients having isolated disc degeneration without accompanying degeneration of the facet joints. However, the presence of significant arthritis of the facet joints is a contraindication for implantation of a disc prosthesis alone. Nevertheless, most patients presenting with severe arthritis of the facet joints have concomitant severe disc degeneration. Surgical treatment of these patients requires either spinal fusion or replacement of both the intervertebral disc and the facet joints. Hitherto, most intervertebral disc prostheses have been designed to be implanted by an anterior replacement procedure through the retroperitoneal approach, while replacement of the facet joints evidently requires a posterior approach.
In order to simplify the surgical procedures required for treating patients by replacement of both the intervertebral disc and the facet joints, various posterior approaches have been proposed. For example, it has been proposed to expose the intervertebral disc through a posterior approach by resecting one or both pairs of facet joints, surgically removing the degenerated disc and replacing it with a suitable prosthesis, and then replacing the facet joints with a suitable prosthesis. In other procedures, the facet joints are spared, which limits the surgical access area for approach to the intervertebral disc typically to a region having a diameter of not more than 1.5 cm. Such limitation, in turn, requires that the size of the prosthesis be relatively small, and has resulted in the use of disc prostheses that are expandable after insertion, or the implantation of two or more small prostheses. The use of such small prostheses may affect the stability of the prosthesis within the intervertebral space, because it is generally considered that the surface area of the contact between the disc prosthesis and the vertebral endplate has to be no less than about 6.5 square centimeters in order to prevent subsidence of the prosthesis.
Hitherto, facet joint prostheses have been anchored to the vertebral bone through one or more of the posterior vertebral structures, i.e., the pedicles, the transverse processes, the spinous process, the laminae, or the inferior articular process. However, such fixation methods must be evaluated by their ability to solve the serious problem of secure fixation of the prosthetic components to the bone. During the physiologic range of motion, the facet joint is under severe shear, bending, and torsional loads. Accordingly, secure fixation of facet joints prostheses to the bone during fatigue loads is a serious challenge.
Accordingly, a need has continued to exist for a prosthesis, implantable through a posterior approach, that could replace both the intervertebral disc and one or both of the facet joints, thus avoiding problems, such as vascular complications, associated with intervertebral disc replacement through the anterior approach.